This invention relates to an engine ignition apparatus and more particularly to an igniter for an internal combustion engine.
FIG. 3 is a circuit diagram illustrating one example of an ignition system for an internal combustion engine. In FIG. 3, reference numeral 1 designates a battery, 2 is a driving transistor connected to the battery 1 through a resistor 3, 4 is a power transistor connected in Darlington connection and driven by an output from the driving transistor 3, 5 is an ignition coil having a primary coil 5a connected between the battery 1 and the collector of the power transistor 4 and a secondary coil 5b connected to a distributor (not shown) which in turn is connected to ignition plugs, 6 is a current detection resistor connected between the emitter of the power transistor 4 and the ground for detecting a current flowing through the primary coil 5a of the ignition coil 5, and 7 is a current regulating circuit for regulating a base current of the power transistor 4 in response to a voltage generated across the current detection resistor 6 so that the current flowing through the primary coil 5a of the ignition coil 5 is controlled.
When the drive transistor 2 is turned off by an external signal, the power transistor 4 turns on to allow an electric current to flow through the primary coil 5a of the ignition coil 5, and when the power transistor 4 turns off, a high voltage generated in on the secondary coil 5b of the ignition coil 5 and is supplied to the ignition plug through the distributor (not shown) to ignite the engine. During this operation, a voltage generated across the resistor 6 due to the current flowing through the primary coil 5a of the ignition coil 5 is supplied to the current regulating circuit 7 which controls the base current of the power transistor 4 in accordance with this voltage to regulate the current flowing through the primary coil 5a of the ignition coil 5 to a predetermined constant value.
FIG. 4 is a schematic plan view illustrating an IC package structure of a known igniter in which the power transistor 4, the current detection resistor 6 and the current regulating circuit 7 electrically connected by conductor wires 12 and 13 are hermetically sealed with a mold resin package 10. The IC package is manufactured by first providing the power transistor 4 and the current regulating circuit 7 on lead frame 11. Then, power transistor 4 and the current regulating circuit 7 are connected by an electrically conductive wire 12 which functions as a current detection portion or a current detection resistor 6, and the power transistor 4, the current regulating circuit 7 are connected to the lead frame 11 by similar conductive wires 13. Then, the above components are hermetically sealed by the mold resin package 10 by the transfer molding.
In the known arrangement illustrated and described in conjunction with FIG. 4, the current detection resistance 6 is provided by means of the resistance of the conductive wire 12 bonded to the power transistor 4 and the current regulating circuit 7 to connect them. Accordingly, the length and the thickness of the conductive wire 12 may vary from one conductive wire to another and the contacting resistance at the bonding connections may be different for each conductive wire, so that the current-regulation value for the current flowing through the primary coil 5a of the ignition coil 5 varies from one igniter to another. If the primary current exceeds the rated current of the power transistor 4 due to a large current, a power transistor which has a large current capacity and which therefore is expensive must be used, and if the primary current is smaller than the predetermined value, the secondary voltage generated at the secondary coil 5b of the ignition coil 5 is not high enough to generate massive sparks.